1. Field of the Invention
This invention relates to accelerator mass spectrometry (AMS), wherein negative ions are formed from a sample to be analyzed and are accelerated in a tandem accelerator to a high-voltage terminal maintained at a high positive potential. A stripper within the high-voltage terminal converts the negative ions to a positive charge state and induces dissociation of all background molecules. After further acceleration and mass analysis, the particles to be analyzed are detected and their properties measured.
2. Description of the Prior Art
During the past twelve years, detection efficiency for long-lived isotopes has been dramatically improved by applying the techniques of Accelerator Mass Spectrometry (AMS). Using AMS, the presence of a radioactive nucleus is detected, not by waiting for it to make a radioactive transformation, but rather by searching for the unstable atoms themselves. The basic principles of AMS instrumentation have been described by Kenneth H. Purser in U.S. Pat. NO. 4,037,100 and by Kenneth H. Purser, R. B. Liebert and C. J. Russo in Radiocarbon 22, (1980) 794.
Measurements of .sup.14 C are good examples of the improvements that are possible using AMS. As a specific example, in a milligram sample of modern carbon (from recent wood or animal products) there are approximately 5.10.sup.7 14 C nuclei. The .sup.14 C nuclei from this milligram disintegrate at a rate of approximately 0.8 disintegrations per hour. In contrast, if using the same sample, AMS procedures were used to count individual .sup.14 C nuclei, it is possible to detect reliably more than 2% of the total .sup.14 C nuclei present and at a rate greater than 150 events/second. Only a few minutes are needed to measure .sup.14 C/.sup.12 C ratios with a precision better than 1%.
A diagram showing the elements of a typical tandem AMS system is shown in FIG. 1. It can be seen that the instrument includes (1) An ion source from which C.sup.- beams can be generated. (2) A tandem accelerator where all the injected ions are simultaneously accelerated to an energy between 2.0 and 2.5 MeV. (3) A gas cell or foil stripper which converts the negative ions to a positive charge state and induces dissociation of all background molecules. (4) A second acceleration stage. (5) A post acceleration mass analysis section where a combination of electric and magnetic deflections eliminates unwanted particles. (6) An ionization detector which independently checks the identity of each arriving particle by measurements of kinetic energy and the rate of energy loss.